Recently, as a result of advancements in various communication media such as communication satellites and recording media, it has become possible to transmit and communicate information over a wide area at high rates of speed. However, as a consequence, it has become easier to access the information and more difficult to keep transmitted information secret.
For example, information such as company communications which are transmitted by communication satellite and internal experimental data are often transmitted on a recording media such as video tape (VT) or optical disk.
The information transmitted in this manner is often in the form of pictures and sound which usually are confidential. This information is often protected by using a scramble encoder having a scrambling function. A conventional scrambling encoder, however, usually requires a large encrypting system and encoder. In addition, the encrypting process is complicated. As a result, an apparatus for recording and reproducing secret information which is encoded in, for example, a VTR is large and expensive.
FIG. 11 is a block diagram of a conventional scrambling encoder. The conventional scrambling encoder includes a computer 101 (CPU) for controlling the scrambling encoder. A variety of computers such as a personal computer or a general-purpose computer may be used for the CPU. In the conventional scrambling encoder, video signals and audio signals are applied to a video scrambler 102 and an audio scrambler 103, respectively. A key signal which is synchronized with the video signal is formed by a key signal forming circuit 104. The key signal is superimposed by a superimposing circuit 105 on the video signal scrambled by the video scrambler 102. The audio signal is frequency-modulated in an FM circuit 106 and mixed with the output of the superimposing circuit 105 in mixing circuit 107 to produce a scrambled composite video and audio signal.
The operation of the conventional scrambling encoder is described below. The video scrambler 102 uses line rotation processing for randomly exchanging the scanning lines of video signals to perform scrambling. The line processing uses a disconnection point of rotation for line rotation which is determined by CPU 101. For example, if the disconnection point is value X, the disconnection point X would be encoded by a key signal (kj). Then, the encoded value X would be formed into a binary signal within the vertical blanking period by the key signal forming circuit. Then, the audio signal is A/D converted and a pseudo-random number sequence pulse signal (PN signal) is added to encode and scramble the audio signal. The initial value of the PN signal is superimposed on the audio signal.
The scrambled signals are descrambled by a scrambling decoder. For example, to descramble the scrambled sound signal, the initial PN signal value is provided to the PN generating circuit. Then, a PN signal series for descrambling is generated, and the scrambled signal is demodulated using the PN signal series. The video signal is descrambled by using the PN signal initial value in every field or specific period to determine the read start position of video signal in the horizontal scanning period. The secrecy of the video and audio signal is maintained by not disclosing the algorithm for encoding the signals.
This conventional scrambling encoder and its corresponding decoder are large and expensive, and difficult to assemble into a television receiver, video tape recorder or video camera.